Aperture synthesis - please explain to a mere mortal

[Jessun]

Imaging theory really - but maybe for the science thread... You tell me. It's about interferometry for imaging using aperture synthesis.

I have ploughed through loads of documentation on the web for an explanation why this works - but most sources presume you're quite read up on things to start with so that initial dummy explanation isn't available anywhere I looked.

I understand roughly what they do, roughly how they hook it up - and how utterly tricky it is. But why does it work?

Sure the distance from the linked up telescopes represents corners of the edges of a large one, but why does simple total aperture and focal lenght theory seem to be set aside?

If I had two little telescopes, it is actually beyond me at the moment why moving them apart a bit does more than collect double the light regardless of what cunning cable I have running between them. The increased resolution is completely lost on me...

The fact is that it works, but how??

[Monki]

Pardon me if I'm completely off here..but in regards to the last paragraph. Maybe its similar to having a pair of eyes. You close one and your FOV is decreased...but when both are open and obviously the correct distance apart (not that it can be altered with eyes anyway hehe) the image is one, but a wider field of view... and in a way you're opening your aperture more to receive more light.

Again...sorry if I'm not in the right track here.

Not sent from a Galaxy S2 on Slim ICS.

[ollypenrice]

I struggled through this on a radio astronomy course since it is a common practice in that world. With two (or more) well separated dishes you synthesize the resolution, but not the light grasp, of a full dish. It's intuitively reasonable that a wide aperture out- resolves a narrow one. What you do is retain this feature of a wide dish (mirror) but you don't get its light grasp. My notes are lost in the UK but if you take your net search into radio astronomy you might find the equations you need.

Olly

[Jessun]

Thanks Monki and Olly. Intuitively though, I see a resolution increase only with increased focal lenght. Is that what might be at work here in some clever way?

I think I'm too thick for equations too...

[Shibby]

The angular resolving power of a telescope is determined only by aperture (ok, and quality of the optics / atmosphere etc). Focal length affects the FOV, but not the resolution.

[Jessun]

A bit confused Lewis. Am I mixing up ordinary image scale - like arc second per pixel - with angular resolving power? If so it's very good since that would derail that train of thought that I got stuck in, and I can try again to get my head around this.

Thanks!

[Shibby]

I think so, yeah. The angular resolution of the optics is determined by the aperture. The resolution of the image you take depends on this and the FL/pixel size combination. Remember there is no real difference between doubling the focal length and halving the pixel dimensions.

[SamAndrew]

Had a read around on wiki, quite interesting, I think the answers you're after are here

http://en.wikipedia.org/wiki/Angular_resolution

and on the related pages around.

It comes down to diffraction patterns, and intereference, so I would have a read about those as well.

[themos]

Yes, interference is key. Radio wave receivers capture the phase information (that is, the actual undulations) of the electromagnetic field, turn it into an alternating current and then can combine currents from different receivers without losing that information. Optical "receivers" can't do this as the oscillation is too fast and even the sensor doesn't capture that. So you have to combine the light beams themselves somewhere, by the time you get to sensor output you've lost the phase information.

[Jessun]

Ok, not saying that I fully understand it but a coin is slowly dropping.

Thanks for the responses guys.

[nytecam]

You can think of Ap-syn as a backyard telescope objective but covered in dirt but on a gigantic scale - these dirty areas, representing the space between individual radio dishes, reduce or dim the signal but not the telescope's resolution.

Resolution relates to the gross diameter of collecting area that the radio dishes occupy. This diameter can be many km or even transcontinental to the earth's diameter. As the earth rotates, during a long observing run, the spacing of the dishes [as seen from afar] changes to fills 'voids' in the collecting area.

The radio signal collected from each dish is saved to disk with an atomic-clock reference signal and the 'images' extracted by 'heavy' computation. The resultant resolution can exceed that of the best optical telescopes:)